For a 100 amp sub panel, you need 3 AWG copper or 1 AWG aluminum wire, protected by a 100-amp double-pole breaker. This assumes 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in the raceway.
- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2)
- Termination Rating: 75°C (Standard for virtually all modern breakers and panel lugs)
- Ambient Temperature: 30°C (86°F)
- Conduit Fill: Maximum 3 current-carrying conductors (no bundling derating applied to base sizes)
- System: 120/240V Single-Phase, 4-wire feeder (2 hots, 1 neutral, 1 ground)
Note: NEC-style guidance is provided here; your local Authority Having Jurisdiction (AHJ) has final authority on all installations.
The Core Ampacity Data: Sizing by the 75°C Column
The most common mistake DIYers make when sizing feeder wire is looking at the 90°C column on the NEC ampacity chart because THHN wire insulation is rated for 90°C. However, NEC 110.14(C) dictates that you must size your wire based on the lowest temperature rating of any connected component. Since standard 100A breakers and subpanel lugs are rated for 75°C, you must use the 75°C column to determine your baseline ampacity.
| AWG Size | Material | 60°C Column (Amps) | 75°C Column (Amps) | 90°C Column (Amps) | Circular Mils (CM) |
|---|---|---|---|---|---|
| 4 AWG | Copper | 70 | 85 | 95 | 41,740 |
| 3 AWG | Copper | 85 | 100 | 115 | 52,620 |
| 2 AWG | Copper | 95 | 115 | 130 | 66,360 |
| 2 AWG | Aluminum | 75 | 90 | 100 | 66,360 |
| 1 AWG | Aluminum | 85 | 100 | 115 | 83,690 |
| 1/0 AWG | Aluminum | 100 | 120 | 135 | 105,600 |
As the table shows, 3 AWG Copper and 1 AWG Aluminum both land exactly on 100 amps in the 75°C column, making them the minimum legal sizes for a 100-amp breaker under standard conditions.
Why 3 AWG Copper and Not 4 AWG?
It is tempting to use 4 AWG copper to save money, as it is rated for 85 amps at 75°C. Some builders attempt to justify this using the NEC 240.4(B) "next size up" rule, which allows you to use the next standard breaker size if the wire ampacity doesn't match a standard breaker. Since 85A is not a standard breaker size, the next size up is 100A.
Why this fails for subpanel feeders: The next-size-up rule only applies if the actual calculated load does not exceed the wire's ampacity (85A). If you are installing a designated "100-amp subpanel," the intent is to allow up to 100 amps of load. If the load reaches 90 amps, a 4 AWG wire will overheat, even though the 100A breaker won't trip. Furthermore, most electrical inspectors will immediately flag a 100A breaker fed by 4 AWG wire during a rough-in inspection because the wire ampacity does not match the breaker rating. Always match the feeder wire ampacity to the breaker size for panel feeders to ensure full utilization and code compliance.
When to Upsize: Voltage Drop and Conduit Derating
The baseline 3 AWG / 1 AWG answer assumes a short run and a single feeder. Real-world jobsite conditions frequently force you to upsize your wire. Here are the two main triggers:
1. Voltage Drop Over Distance
The NEC recommends a maximum voltage drop of 3% for feeders to ensure efficient operation. Using the standard voltage drop formula ($VD = \frac{2 \times K \times I \times D}{CM}$) and assuming a realistic 80A continuous load on a 240V circuit:
- At 100 feet (3 AWG Cu): Voltage drop is ~3.9V (1.6%). Passes.
- At 150 feet (3 AWG Cu): Voltage drop is ~5.8V (2.4%). Passes.
- At 200 feet (3 AWG Cu): Voltage drop is ~7.8V (3.25%). Fails the 3% threshold.
If your conduit run exceeds roughly 180 feet, you must upsize to 2 AWG Copper or 1/0 AWG Aluminum to maintain power quality. You can verify your specific run using tools like the Southwire Voltage Drop Calculator.
2. Bundling Derating (The 90°C Column Saves You)
If you pull more than three current-carrying conductors in a single conduit (for example, running two separate subpanel feeders in the same PVC pipe), NEC 310.15(C)(1) requires you to derate the wire's ampacity. For 4-6 conductors, you multiply the ampacity by 80%.
Here is where the 90°C column is legally useful: Derating is calculated from the 90°C column, not the 75°C column.
3 AWG Copper at 90°C is 115A.
115A × 0.80 (derating factor) = 92A.
Because 92A is less than the 100A breaker, 3 AWG is no longer legal if bundled. You must step up to 2 AWG Copper (130A at 90°C × 0.80 = 104A), which safely clears the 100A requirement.
Copper vs. Aluminum: Cost, Termination, and Reality
Aluminum is significantly cheaper and lighter than copper, but it requires stricter installation practices. Here is how they compare for a 100A feeder:
| Criterion | 3 AWG Copper (THHN) | 1 AWG Aluminum (XHHW-2) |
|---|---|---|
| Approximate Cost (per foot) | $3.50 - $4.50 | $1.20 - $1.80 |
| Wire Stiffness | Moderate; requires firm bending | Highly flexible; easier to pull in conduit |
| Termination Prep | Strip and land directly | Must brush with anti-oxidant paste (e.g., Noalox) |
| Torque Requirements | Standard calibrated torque screwdriver | Strict adherence to lug torque specs; prone to creep if under-torqued |
| Lug Compatibility | Fits standard 100A breaker lugs | May require larger lug or specific AL-rated breaker lugs |
Never skip the anti-oxidant compound when terminating aluminum wire. Bare aluminum oxidizes rapidly when exposed to air, creating a high-resistance layer that generates intense heat under load. Furthermore, NEC 110.14(D) mandates the use of a calibrated torque tool for all terminations. Aluminum is highly susceptible to thermal expansion and "cold creep," meaning an under-torqued or over-torqued lug will eventually loosen and cause a fire hazard.
When an Engineer or the AHJ Must Confirm
While the 3 AWG Copper / 1 AWG Aluminum rule covers 90% of residential and light commercial subpanel installations, you must step back and consult a licensed professional engineer or your local electrical inspector under the following conditions:
- Service Entrance vs. Feeder: If this 100A panel is a main service disconnect rather than a subpanel fed from another panel, utility rules and NEC Article 230 apply, which may dictate specific grounding, bonding, and physical protection requirements.
- High Ambient Temperatures: If the conduit runs through an attic or boiler room where ambient temperatures regularly exceed 30°C (86°F), you must apply temperature correction factors from NEC Table 310.15(B)(1), which will likely force an upsize to 2 AWG Copper.
- Continuous Industrial Loads: If the subpanel will supply continuous loads (operating for 3 hours or more) that total exactly 100 amps, the NEC requires the overcurrent device and conductors to be sized at 125% of the continuous load (125A), completely changing the wire size requirement.
- Local Amendments: Some municipalities have strict local amendments that ban aluminum feeders entirely or require larger grounding conductors than NEC Table 250.122 specifies.
By starting with the 75°C ampacity column, verifying your voltage drop over the actual conduit distance, and checking for bundling derating, you ensure your 100-amp subpanel is safe, efficient, and ready for inspection.
